A screw thread call-out is a standardized way to describe the threading on a fitting or cap—especially those using the English (imperial) measurement system.

What Does a Thread Call-Out Look Like?

A typical thread call-out includes two numbers, such as 1/4-28:

• 1/4 refers to the outer diameter of the thread (in inches).
• 28 refers to the number of threads per inch (TPI)—also known as the thread pitch.

So, a 1/4-28 fitting has a 1/4-inch diameter and 28 threads per inch. The higher the second number, the finer the threads.

Why Is This Important?

To ensure a proper seal and avoid leaks, the thread call-out of your fitting or cap must match the port it connects to. For example:

A 1/4-28 male fitting must be paired with a 1/4-28 female port.

What About Smaller Fittings?

For fittings smaller than 1/4 inch, the diameter is often given in gauge size rather than inches. For example:

• A 10-32 thread means:
  • 10 gauge diameter
  • 32 threads per inch

This is a common size for small HPLC fittings.

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🔧 Click HERE for Instructions on Properly Installing Screw Caps